Caprolactone is a cyclic ester commonly used as a starting material in the preparation of oligomers and polymers.
One method of preparing caprolactone involves the Baeyer-Villiger oxidation cyclohexanone in the presence of a peracid such as peracetic acid. This method is described, for example, in Organic Reactions Volume 43, pages 251-798 (1993).
U.S. Pat. No. 3,189,619 discloses another method in which a 6-hydroxycaproic ester is reacted in a first stage with a trialkyl borate and the reaction product is then converted into caprolactone by heating at from 200° C. to 250° C. under reduced pressure. In another process, disclosed in French Patent 1,474,903, a 6-hydroxycaproic ester is heated to 150° to 350° C. in the presence of an oxide, such as magnesium oxide, zinc oxide, cadmium oxide, alumina or titanium dioxide, in the liquid phase under reduced pressure, and caprolactone is distilled off continuously.
U.S. Pat. No. 5,068,361 discloses a process for the preparation of caprolactone in which a 6-hydroxycaproic ester is heated at from 150° C. to 450° C. in the presence of an oxidic catalyst wherein 6-hydroxycaproic ester vapor is passed, together with an inert carrier gas, over a fixed-bed or fluidized-bed oxidic catalyst.
At col. 1, line 53 to 57, the patentees disclosed that:                6-Hydroxycaproic esters are advantageously vaporized at from 180° C. to 300° C. It has proven advantageous if solvents which are inert under the reaction conditions, such as esters, e.g., dioxane or tetrahydrofuran, are also vaporized simultaneously. Advantageously, from 50 to 90% strength by weight solutions of 6-hydroxycaproic esters in such solvents are used.        
However, the use of organic solvents such as, for example, dioxane or tetrahydrofuran can pose health risks to humans and contaminate the environment. Thus, appropriate precautions must be taken to avoid such health risks and environmental contamination.
Furthermore, a by-product of the conversion of 6-hydroxycaproic esters to caprolactone by the above process is the corresponding alcohol. For example, each mole of 6-methoxycaproic acid converted to caprolactone, one mole of methanol is produced. The by-product alcohols are potentially harmful to the environment and must be recovered for disposal.
New processes for the manufacture of caprolactone, and other lactones, are desired which can limit or substantially eliminate the use of potentially harmful solvents and alcohol by-products.